All industries that roll flat products such as paper, plastic, steel or other metal sheets must contour the roll surface in order to obtain an acceptable flatness or profile of the product. The most common approach has been the grinding of a crown onto the rolls which compensates for the composite roll deflections under load. This method, unfortunately, is effective only for a fixed loading and a unique product size.
It is well known that in the rolling of metals between rolls which themselves are hardened metal, usually steel, heat affects the crown of the rolls. In hot rolling most of the heat imparted to the rolls is derived from their contact with the hot metal but even in cold rolling the working of the metal generates heat, some of which is transferred to the rolls. As the roll heats its crown increases. The change in crown is measured in hundredths of a millimeter but in the rolling of strip of a few hundredths of a millimeter thickness changes in crown alter not only the gauge of the strip but its flatness or shape.
Radial heat flow in a steel roll is of course quite slow. A 1500 millimeter diameter back-up roll which has reached a steady state temperature of 120 degrees C. will require more than an hour for an 0.08 millimeter radial crown change and a 750 millimeter diameter work roll would require about half an hour for the same change. It is well known to spray a liquid coolant on a roll to control its crown but the thermal inertia of conventional rolls severely limits the usefulness of that expedient.
In addition to gauge and shape the strip producer is also concerned with the effect known as "edge drop", or feathering, which is another aspect of roll crowning. A strip mill operator is required to roll on the same mill strip of various widths as the customer orders. A perfectly flat strip from edge-to-edge could only be obtained, prior to my invention to be described hereinafter, when the strip is the same width as the mill. When narrower strip had to be rolled the pair roll ends extending beyond the strip on each side were forced together producing a tapered edge on the strip extending inwardly from perhaps 25 millimeters to as much as 100 millimeters in some cases. That edge had to be trimmed off in a later operation so reducing the yield from the mill. One method of dealing with that problem is suggested in Feldmann, et al. U.S. Pat. No. 4,479,374 of Oct. 30, 1984. A roll stand is there disclosed which has lathe-type tools adjusted to trim down the ends of the work roll projecting beyond the strip. With such a mill rolling must be scheduled so that all wide strip orders are rolled before narrow strip orders are rolled.